The present invention relates to porous extensible polymeric webs. In particular, the present invention relates to macroscopically-expanded three-dimensional apertured polymeric webs.
It has long been known in the field of disposable absorbent articles that it is desirable to construct absorptive devices, such as disposable diapers, sanitary napkins, incontinent briefs, bandages, wound dressings, and the like, with elastic elements to improve the range of size, ease of motion, and sustained fit. It is also well known that it is preferable, especially in such products intended to be worn in hot and humid conditions, to provide adequate porosity to all areas of the article where undue occlusion of the skin may cause sensitized skin or heat rash. Due to the nature of many disposable absorbent articles there is a high potential for skin irritation due to trapping of moisture and other body exudates between the elasticized portion of the article and the skin of the wearer. Elasticized portions of disposable articles are particularly prone to causing skin irritations as they tend to be more conformable to the body, and therefore more likely to occlude areas of the skin, often for long periods of time. Various methods are known in the art for imparting elasticity to polymer films, and various methods are known in the art for imparting porosity to polymer films, but there remains a need for a polymeric film or web that provides for both adequate elasticity and porosity, such as may be adapted for durable, prolonged use in garments, particularly disposable garments.
Disposable diapers and other absorbent articles fitted with elasticized leg cuffs or elasticized waist bands for a more comfortable fit, as well as providing for better leakage control, are known in the art. Often, the elasticity is accomplished with a heat treatment of polymeric materials that results in a desirable shirring or gathering of a portion of the diaper. One such method of treatment is disclosed in U.S. Pat. No. 4,681,580, issued to Reising et al. on Jul. 21, 1987, and hereby incorporated by reference herein.
An improved method for sequentially stretching a xe2x80x9czero strainxe2x80x9d stretch laminate web to impart elasticity thereto is disclosed in U.S. Pat. No. 5,143,679 issued to Weber et al. on Sep. 1, 1992 and hereby incorporated herein by reference. Weber ""679 teaches the use of a stretch laminate material formed of at least two plies, one of which is stretchable and elastomeric, while the second ply is elongatable, but not necessarily elastomeric. The plies are either intermittently or substantially continuously secured to one another along at least a portion of their coextensive surfaces while in a substantially untensioned (xe2x80x9czero-strainxe2x80x9d) condition. Weber ""679 further discloses an improved method and apparatus for sequentially stretching the xe2x80x9czero-strainxe2x80x9d stretch laminate portions of the web during the incremental stretching process to impart elasticity in the direction of stretching without rupturing the laminate web in the process. Further improvements are taught in U.S. Pat. No. 5,156,793 issued to Buell et al. on Oct. 20, 1992 and U.S. Pat. No. 5,167,897 issued to Weber et al. on Dec. 1, 1992, both of which are hereby incorporated by reference herein.
Elasticized polymeric webs may also be produced from elastomeric materials known in the art, and may be laminates of polymeric materials such as disclosed in U.S. Pat. No. 5,501,679, issued to Krueger et al. on Mar. 26, 1996. Laminates of this type are generally prepared by coextrusion of elastomeric materials and inelastic skin layers followed by stretching the laminate past the elastic limit of the skin layers and then allowing the laminate to recover. Elastomeric webs or films such as those described above may be used in the body hugging portions of garments, such as the waistband portions and leg cuffs, but are generally not porous enough to prevent undesirable skin irritations when used for prolonged periods of time.
Several means of rendering elasticized planar polymer films more porous are known in the art, such as die punching, slitting, and hot-pin melt aperturing. However, when any of the above techniques is applied to thermoplastic elastomeric films, the increase in porosity is accompanied by a decrease in the degree of reliable elastic performance. For example, in the case of circular apertures in a planar film, it is well known that for an applied stress S1, a resultant local stress, S2, is created orthogonal to the applied stress about the apertures. This local stress, S2, is greater than S1, approaching a magnitude up to 3 times the applied stress. For non-round apertures the concentration of stress can be even greater. As a result, apertures become sources of tear initiation sites at their edges, because the edges of the material form the edges of the apertures in the plane of applied stress. For common thermoplastic elastic films, such apertures facilitate tear initiation which can propagate over time leading to catastrophic failure of the film. When used in elasticized portions of disposable absorbent articles, this failure results in the loss of important elastic characteristics, including loss of comfort, fit and use of the absorbent article.
Prior art web structures that do provide adequate porosity so as to be preferable for use as the wearer-contacting surface on disposable absorbent articles have been of two basic varieties, i.e., inherently fluid-pervious structures, such as fibrous nonwovens, and fluid-impervious materials such as polymeric webs which have been provided with a degree of fluid permeability via aperturing to permit fluid and moisture flow therethrough. Neither variety is characteristically elastic, and as a result both are generally used in regions of an absorbent article requiring fluid permeability but not extensibility, such as the body-contacting layer of a catamenial pad.
Commonly assigned U.S. Pat. No. 3,929,135 issued to Thompson on Dec. 30, 1975, and hereby incorporated herein by reference, suggests a suitable body-contacting porous polymeric web for disposable articles. Thompson teaches a macroscopically-expanded, three-dimensional topsheet comprised of liquid-impermeable polymeric material. However, the polymeric material is formed to comprise tapered capillaries, the capillaries having a base opening in the plane of the topsheet, and an apex opening in intimate contact with the absorbent pad utilized in the disposable absorbent bandage. The polymer material taught by Thompson is not generally an elastomer, however, and Thompson depends on the inelastic properties of the heat-molded single layer film to produce the desired three-dimensional structure.
Still another material which has been utilized as a body contacting surface in a disposable absorbent article context is disclosed in commonly assigned U.S. Pat. No. 4,342,314 issued to Radel et al. on Aug. 3, 1982, and hereby incorporated herein by reference. The Radel et al. patent discloses an improved macroscopically-expanded three-dimensional plastic web comprising a regulated continuum of capillary networks originating in and extending from one surface of the web and terminating in the form of apertures in the opposite surface thereof. In a preferred embodiment, the capillary networks are of decreasing size in the direction of liquid transport.
The macroscopically-expanded three-dimensional plastic webs of the type generally described in the aforementioned commonly assigned Thompson and Radel et al. patents have met with good success in permitting adequate liquid permeability due to the porosity provided by the apertures. However, because of material limitations such webs do not generally possess the requisite elasticity to allow the resulting web to have significant elastomeric characteristics. This shortcoming substantially limits the use of such webs in elasticized portions of an absorbent article. Further, when such webs are extended in one or more directions, the open area provided by the apertures, generally reduces. This can significantly reduce the air and moisture permeability of the web which can increase skin irritation, especially when the web is located in a high stretch region of an article, such as a diaper.
Accordingly, it would be desirable to provide a breathable apertured elastomeric web designed to dissociate the effects of an applied strain on the web from the edges of the apertures and hence retard or prevent the onset of tear initiation. More particularly, in a particularly preferred embodiment, it would be desirable to provide a breathable macroscopically-expanded three-dimensional apertured elastomeric web that is able to substantially recover its three-dimensional shape after being subjected to an applied strain of up to about 400% or more. It would also be desirable to provide a breathable macroscopically-expanded three-dimensional apertured web including apertures having a major axis and a minor axis perpendicular to the major axis wherein the major axis is oriented generally orthogonal to the expected direction of elongation of the web during use.
In a preferred embodiment, the present invention pertains to a macroscopically-expanded, three-dimensional, elastomeric web suitable for use in elasticized portions of disposable absorbent articles such as bandages, diapers and pull-up diaper training pants. In a preferred embodiment the web has a continuous first surface and a discontinuous second surface remote from the first surface. An elastomeric web of the present invention preferably comprises a formed film having at least two polymeric layers, with at least one layer being an elastomer and at least one layer being a substantially less elastomeric skin layer. In a preferred embodiment the elastomeric web exhibits a multiplicity of primary apertures in the first surface of the web, the primary apertures being defined in the plane of the first surface by a continuous network of interconnecting members, each interconnecting member exhibiting an upwardly concave-shaped cross-section along its length. In a preferred embodiment each interconnecting member exhibits a generally U-shaped cross-section along a portion of its length, the cross-section comprising a base portion generally in the plane of the first surface of the web and sidewall portions joined to each edge of the base portion and interconnected with other sidewall portions. The interconnected sidewall portions extend generally in the direction of the second surface of the web, and are interconnected to one another intermediate the first and the second surfaces of the web. The interconnected sidewall portions terminate substantially concurrently with one another to form a secondary aperture in the plane of the second surface of the web.
Also disclosed is a method of producing the elastomeric web of the present invention comprising providing a multilayer elastomeric film, supporting the film on a forming structure, and applying a fluid pressure differential across the thickness of the multilayer film. The fluid pressure differential is sufficiently great to cause the multilayer film to conform to the supporting structure and rupture in at least portions of the formed film.
When used as an extensible, porous member in an absorbent article, the elastomeric layer of the present invention allows the interconnecting members to stretch in the plane of the first surface. The three-dimensional nature of the web places the secondary apertures in a plane of the second surface remote from the plane of the first surface, initially removing web stresses from the tear initiation sites at the edges of the secondary apertures. Initial web strain results in the base of the interconnecting members experiencing strain in the first surface. As the web strain increases, the sidewall portions of the interconnecting members intermediate the first and second surface experience strain as they begin to approach the plane of the first surface. Ultimately, upon adequate web strain, the plane of the second surface approaches the plane of the first surface and the edges of the secondary apertures will experience web strain as well.
Therefore, the three-dimensional nature of the web allows the strain on the interconnecting members in the plane of the first surface to be dissociated from the strain at the secondary apertures in the secondary surface, and therefore decoupled from potential strain-induced stress at tear initiation sites. This dissociation, or decoupling, of the strain-induced stress of the web from strain-induced stress at the secondary apertures significantly increases web reliability by allowing repeated and sustained web strains of up to about 400% or more without failure of the web due to tear initiation at the apertures.